194 A HISTORY OF BIRDS 



But Dr. Wallace also implies that birds employ a certain 

 amount of reasoning in their building, or rather, perhaps, that 

 they act on suggestion, on the stimulus of their environment. 

 That when impelled to build they seize on the materials nearest 

 at hand. Thus the Wren " frequenting hedgerows and low 

 thickets, builds its nest generally of moss, a material always 

 found where it lives. ..." " Rooks dig in pastures and ploughed 

 fields for grubs, and in doing so must continually encounter 

 roots and fibres. These are used to line its nest." But what 

 about the foundation of the nest ? And again : " Swallows use 

 clay and mud from the margins of the ponds and rivers over 

 which they find their insect food. The materials of birds' nests 

 . . . are, then, those which come first to hand." But the Sand- 

 martin is equally partial to streams, yet this bird does not build 

 a nest of mud, but laboriously drives tunnels in sandbanks and 

 lines the further chamber with feathers ! Finally, he remarks : 

 " The clumsy hooked bills, short necks and feet and heavy 

 bodies of Parrots, render them quite incapable of building a 

 nest. ..." But the Quaker-parrot {Myopsittacus monachus) 

 builds a large domed nest of sticks, and is, moreover, no mean 

 craftsman at weaving. 



The origin of nest-building, as we have suggested (p. 174), 

 began probably with the ground-builders, which hit upon the 

 device of collecting twigs or grass to form a dry bed while 

 brooding, thereby preserving their eggs, while those less 

 intelligent failed to rear offspring. The young of these more 

 intelligent parents, inheriting this variation in the direction of 

 increased intelligence, would, stimulated by the sexual impulse 

 at the proper season, repeat the same tactics. With changes in 

 the environment new needs arose and were similarly met by at 

 least a few individuals, and thus, by slow increments the gradual 

 evolution of complex nests arose. Even to-day many stages 

 can be traced in the growth and perfection of the more complex 

 nests of any particular type, such as the pendant nests of the 

 Weaver-birds, and Hangnests, for example (p. 183). While, as in 

 the case of the Cliff-swallows (p. 185), we may even have a rever- 

 sion of the older type of nests, inasmuch as when suitable holes 

 are to be found a mud-rim around the entrance is all that is con- 

 structed, the typical retort-shaped nest growing in inverse pro- 

 portion to the size of the hole. 



